The present invention relates to a display device and a driving method thereof, and particularly, to a display device and a driving method thereof suitable for gradation display of an organic electroluminescent device or an organic EL element.
As mobile computing has become popular, demands for a flat type display apparatus have increased. As a flat type display apparatus, a liquid crystal display apparatus has been conventionally used. However, a liquid crystal display apparatus has a problem that the view angle is narrow and the response characteristic is low.
In contrast, attention has recently been paid to a display apparatus which achieves display by light emission with use of an organic electroluminescent element, as another flat type display apparatus which has a wide view angle and an excellent response characteristic. Pixels of an organic electroluminescent panel used in such a display apparatus are respectively constructed by organic EL elements 31, drive transistors 32 consisting of TFTs (thin film transistors), and selection transistors 33 also consisting of TFTs, as shown in FIG. 32. Further, the gate of the selection transistor 33 is connected to a gate line GL connected to a gate driver (not shown), and the drain of the selection transistor 33 is connected to a drain line DL connected to a drain driver (not shown). The source of the selection transistor 33 is respectively connected to the gate of the drive transistor 32. The source of the drive transistor 32 is connected to the cathode of corresponding organic EL element 31, and the drain thereof is grounded. The anodes of all the organic EL elements 31 are connected to be always supplied with a constant voltage of a reference potential Vdd.
In case where a full-color image is displayed on the organic EL display apparatus, light-emission luminance gradation display of each organic EL element 31 is achieved by controlling each of voltages applied through the drain line DL and the selection transistor 33 to the drive transistor 32 from the drain driver, thereby to control a source-drain current of the drive transistor 32.
Specifically, as is shown in the characteristic graph in FIG. 33, with the reference potential Vdd kept set to be constant, i.e., with the source-drain voltage Vsd of the drive transistor 32 kept set to be constant, the drain current Isd between the source and drain of the drive transistor 32 is changed by changing the gate voltage Vg of the drive transistor 32. In this manner, the amount of current flowing through the organic EL element 31 changes thereby changing the energy excited when holes and electrons are respectively coupled with each other in the organic EL layer of the organic EL element 31. As a result, the amount of light emitted by the organic EL element 31 changes. The gate voltage Vg of the drive transistor 32 is changed by a change of the drain signal voltage applied to the drain of a selection transistor 33.
It is very difficult that the gate voltages and the source-drain currents of the drive transistors 32 connected to all the organic EL elements 31 in a panel are provided with uniform characteristics, according to increase of number of pixels. Therefore, the source-drain current varies even if the gates of the drive transistors 32 are applied with a voltage of an equal value. As a result, the values of currents flowing through organic EL elements 31 greatly vary, or in other words, the amounts of the holes and the electrons greatly vary, so that the amount of light emitted by an organic EL element 31 varies for every pixel even though one same signal is outputted to drain lines DL, resulting in that the quality of an image displayed on the organic EL panel is degraded.
The problem described above leads to another problem that the yield of organic EL panels is lowered.